The long-term goal of the proposed research is to identify and functionally characterize the genes regulating intramolluscan larval development in the human blood fluke Schistosoma mansoni. Schistosome development within the snail intermediate host is complex, involving several stages including the snail-infective miracidium, mother sporocyst, daughter sporocyst, and finally the human-infective cercaria. Because the miracidium -to-mother sporocyst transition is assumed to involve numerous physiological adaptations going from its free-living to parasitic existence, it is hypothesized that such a transition is regulated by critical, stage-specific changes in gene expression. Similarly, the initiation of asexual reproduction (embryogenesis) in mother sporocysts to give rise to multiple daughter sporocysts and within daughters, the formation of cercariae, is presumed to involve developmental processes orchestrated through the differential expression of genes in a stage-specific fashion. However, to date, very little is known regarding the identity of genes expressed in molluscan larval stages, especially the asexually reproducing sporocysts, and there exists no information on changes in gene expression profiles associated with sequential development from the miracidium to mother sporocyst to daughter sporocyst stages. To address this critical information gap, and to begin investigating the hypothesis that larval development is driven by defined stage-specific changes in gene expression, the following specific aims are proposed: (1) to evaluate gene expression profiles for miracidia and mother sporocysts (early-and late-developing) under in vitro culture conditions by employing S. mansoni oligonucleotide gene microarrays; (2) to confirm and validate in vitro DNA microarray results by measuring expression of specific genes in mother sporocysts developing under in vivo infections conditions; (3) to investigate the effects of snail host factors on larval schistosome gene expression using schistosome gene microarray analyses, followed by identification and characterization of larval expression-modulating snail molecules; and (4) to evaluate whether specific genes found to be upregulated in a larval stage-associated manner are functionally tied to defined developmental phenotypes using double-stranded RNA interference methods. It is anticipated that the proposed microarray profiling approach will fill a critical information gap regarding S. mansoni larval stage-associated gene expression, and will provide important insights into the role of specific genes in regulating the developmental process. Results of this work may lead to novel strategies for controlling infections within the snail host by disrupting specific larval development-dependent genes or their products.